198 lines
7.3 KiB
Go
198 lines
7.3 KiB
Go
// Copyright 2016 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package tar
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// Constants to identify various tar formats.
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const (
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// The format is unknown.
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formatUnknown = (1 << iota) / 2 // Sequence of 0, 1, 2, 4, 8, etc...
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// The format of the original Unix V7 tar tool prior to standardization.
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formatV7
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// The old and new GNU formats, which are incompatible with USTAR.
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// This does cover the old GNU sparse extension.
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// This does not cover the GNU sparse extensions using PAX headers,
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// versions 0.0, 0.1, and 1.0; these fall under the PAX format.
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formatGNU
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// Schily's tar format, which is incompatible with USTAR.
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// This does not cover STAR extensions to the PAX format; these fall under
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// the PAX format.
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formatSTAR
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// USTAR is the former standardization of tar defined in POSIX.1-1988.
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// This is incompatible with the GNU and STAR formats.
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formatUSTAR
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// PAX is the latest standardization of tar defined in POSIX.1-2001.
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// This is an extension of USTAR and is "backwards compatible" with it.
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//
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// Some newer formats add their own extensions to PAX, such as GNU sparse
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// files and SCHILY extended attributes. Since they are backwards compatible
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// with PAX, they will be labelled as "PAX".
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formatPAX
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)
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// Magics used to identify various formats.
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const (
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magicGNU, versionGNU = "ustar ", " \x00"
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magicUSTAR, versionUSTAR = "ustar\x00", "00"
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trailerSTAR = "tar\x00"
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)
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// Size constants from various tar specifications.
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const (
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blockSize = 512 // Size of each block in a tar stream
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nameSize = 100 // Max length of the name field in USTAR format
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prefixSize = 155 // Max length of the prefix field in USTAR format
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)
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var zeroBlock block
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type block [blockSize]byte
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// Convert block to any number of formats.
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func (b *block) V7() *headerV7 { return (*headerV7)(b) }
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func (b *block) GNU() *headerGNU { return (*headerGNU)(b) }
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func (b *block) STAR() *headerSTAR { return (*headerSTAR)(b) }
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func (b *block) USTAR() *headerUSTAR { return (*headerUSTAR)(b) }
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func (b *block) Sparse() sparseArray { return (sparseArray)(b[:]) }
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// GetFormat checks that the block is a valid tar header based on the checksum.
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// It then attempts to guess the specific format based on magic values.
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// If the checksum fails, then formatUnknown is returned.
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func (b *block) GetFormat() (format int) {
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// Verify checksum.
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var p parser
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value := p.parseOctal(b.V7().Chksum())
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chksum1, chksum2 := b.ComputeChecksum()
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if p.err != nil || (value != chksum1 && value != chksum2) {
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return formatUnknown
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}
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// Guess the magic values.
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magic := string(b.USTAR().Magic())
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version := string(b.USTAR().Version())
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trailer := string(b.STAR().Trailer())
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switch {
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case magic == magicUSTAR && trailer == trailerSTAR:
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return formatSTAR
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case magic == magicUSTAR:
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return formatUSTAR
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case magic == magicGNU && version == versionGNU:
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return formatGNU
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default:
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return formatV7
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}
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}
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// SetFormat writes the magic values necessary for specified format
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// and then updates the checksum accordingly.
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func (b *block) SetFormat(format int) {
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// Set the magic values.
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switch format {
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case formatV7:
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// Do nothing.
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case formatGNU:
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copy(b.GNU().Magic(), magicGNU)
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copy(b.GNU().Version(), versionGNU)
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case formatSTAR:
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copy(b.STAR().Magic(), magicUSTAR)
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copy(b.STAR().Version(), versionUSTAR)
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copy(b.STAR().Trailer(), trailerSTAR)
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case formatUSTAR, formatPAX:
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copy(b.USTAR().Magic(), magicUSTAR)
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copy(b.USTAR().Version(), versionUSTAR)
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default:
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panic("invalid format")
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}
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// Update checksum.
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// This field is special in that it is terminated by a NULL then space.
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var f formatter
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field := b.V7().Chksum()
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chksum, _ := b.ComputeChecksum() // Possible values are 256..128776
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f.formatOctal(field[:7], chksum) // Never fails since 128776 < 262143
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field[7] = ' '
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}
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// ComputeChecksum computes the checksum for the header block.
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// POSIX specifies a sum of the unsigned byte values, but the Sun tar used
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// signed byte values.
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// We compute and return both.
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func (b *block) ComputeChecksum() (unsigned, signed int64) {
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for i, c := range b {
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if 148 <= i && i < 156 {
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c = ' ' // Treat the checksum field itself as all spaces.
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}
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unsigned += int64(uint8(c))
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signed += int64(int8(c))
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}
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return unsigned, signed
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}
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type headerV7 [blockSize]byte
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func (h *headerV7) Name() []byte { return h[000:][:100] }
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func (h *headerV7) Mode() []byte { return h[100:][:8] }
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func (h *headerV7) UID() []byte { return h[108:][:8] }
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func (h *headerV7) GID() []byte { return h[116:][:8] }
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func (h *headerV7) Size() []byte { return h[124:][:12] }
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func (h *headerV7) ModTime() []byte { return h[136:][:12] }
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func (h *headerV7) Chksum() []byte { return h[148:][:8] }
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func (h *headerV7) TypeFlag() []byte { return h[156:][:1] }
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func (h *headerV7) LinkName() []byte { return h[157:][:100] }
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type headerGNU [blockSize]byte
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func (h *headerGNU) V7() *headerV7 { return (*headerV7)(h) }
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func (h *headerGNU) Magic() []byte { return h[257:][:6] }
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func (h *headerGNU) Version() []byte { return h[263:][:2] }
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func (h *headerGNU) UserName() []byte { return h[265:][:32] }
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func (h *headerGNU) GroupName() []byte { return h[297:][:32] }
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func (h *headerGNU) DevMajor() []byte { return h[329:][:8] }
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func (h *headerGNU) DevMinor() []byte { return h[337:][:8] }
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func (h *headerGNU) AccessTime() []byte { return h[345:][:12] }
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func (h *headerGNU) ChangeTime() []byte { return h[357:][:12] }
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func (h *headerGNU) Sparse() sparseArray { return (sparseArray)(h[386:][:24*4+1]) }
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func (h *headerGNU) RealSize() []byte { return h[483:][:12] }
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type headerSTAR [blockSize]byte
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func (h *headerSTAR) V7() *headerV7 { return (*headerV7)(h) }
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func (h *headerSTAR) Magic() []byte { return h[257:][:6] }
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func (h *headerSTAR) Version() []byte { return h[263:][:2] }
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func (h *headerSTAR) UserName() []byte { return h[265:][:32] }
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func (h *headerSTAR) GroupName() []byte { return h[297:][:32] }
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func (h *headerSTAR) DevMajor() []byte { return h[329:][:8] }
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func (h *headerSTAR) DevMinor() []byte { return h[337:][:8] }
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func (h *headerSTAR) Prefix() []byte { return h[345:][:131] }
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func (h *headerSTAR) AccessTime() []byte { return h[476:][:12] }
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func (h *headerSTAR) ChangeTime() []byte { return h[488:][:12] }
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func (h *headerSTAR) Trailer() []byte { return h[508:][:4] }
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type headerUSTAR [blockSize]byte
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func (h *headerUSTAR) V7() *headerV7 { return (*headerV7)(h) }
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func (h *headerUSTAR) Magic() []byte { return h[257:][:6] }
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func (h *headerUSTAR) Version() []byte { return h[263:][:2] }
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func (h *headerUSTAR) UserName() []byte { return h[265:][:32] }
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func (h *headerUSTAR) GroupName() []byte { return h[297:][:32] }
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func (h *headerUSTAR) DevMajor() []byte { return h[329:][:8] }
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func (h *headerUSTAR) DevMinor() []byte { return h[337:][:8] }
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func (h *headerUSTAR) Prefix() []byte { return h[345:][:155] }
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type sparseArray []byte
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func (s sparseArray) Entry(i int) sparseNode { return (sparseNode)(s[i*24:]) }
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func (s sparseArray) IsExtended() []byte { return s[24*s.MaxEntries():][:1] }
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func (s sparseArray) MaxEntries() int { return len(s) / 24 }
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type sparseNode []byte
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func (s sparseNode) Offset() []byte { return s[00:][:12] }
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func (s sparseNode) NumBytes() []byte { return s[12:][:12] }
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